In operation, an electron beam from an electron gun creates ions along its path from residual gases in the device when molecules of the gases are hit by the electron beam. Positive ions created by the electron beam can drift into the gun electrode region where they are likely to be focused by the potential of the gun electrodes, accelerated towards and impact against the cathode.
The life of a cathode in any electron injector system is restricted by evaporation and sputtering effects. If the cathode is bombarded with high energy ions, the cathode material is sputtered creating damage which limits the effective operational life of the cathode. Therefore, ion traps have been used in many systems to capture the ions before they enter into a gun electrode acceleration region. This can be done by placing a plate in the path of the ions so that the ions will collide with the plate rather than drift into the gun electrode region. In applications where the electrons must travel along the same path as the ions they have created, but in the opposite direction, the ions must be deflected onto the plate by other means such as electrostatically.
U.S. Pat. No. 4,743,794 illustrates one type of ion trap for a cathode ray tube. In this type of tube, a semiconductor cathode has an annular emitting region surrounding a central axis of the tube. A first electrode grid adjacent the cathode has an opening which extends sufficiently far from the axis to pass electrons emitted from the annular region. A further electrode, a screen grid, is located farther from the cathode and is operated at a higher potential than the first electrode forming a positive lens which converges the annular electron beam into a cross-over at approximately the area of an opening in the screen grid through which the electron beam travels. The opening in the screen grid has a smaller diameter than the annular emitting region so that any positive ions generated near the axis and pass through the opening in the screen grid would be accelerated parallel to the axis and strike the central area of the cathode. They would not strike the annular emitting region. Furthermore, positive ions which are generated farther away from the axis would be accelerated away from the axis and the opening in the screen grid. This design prevents the majority of positive ions which are generated in the tube from impacting against the annular emitting region of the cathode. The only positive ions which will strike the annular emitting region are ones generated in a small region between the cathode and the first electrode. However, these particular ions have a relatively low energy so very little damage is done to the emitting region.
U.S. Pat. No. 3,586,901 illustrates another type of ion trap to reduce the rate at which positive ions bombard electron gun cathodes. This particular ion trap contains a pair of closely spaced titanium anodes in tandem with apertures through which the electron beam from the cathode travel. The anode nearest the cathode has a smaller aperture and is operated at a higher positive potential with respect to the cathode than the other anode. This creates a potential hill and each positive ion formed outside the region between the cathode and anodes would have to overcome that hill in order to be attracted to the cathode.
U.S. Pat. No. 4,720,832 describes another type of device to prevent impurity ions from contaminating an optical window in a laser. The device in this U.S. Patent comprises pairs of magnets on each side of the optical axis which apply magnetic fields to charged particles moving along the axis towards the window so as to deflect them away from the axis. A number of annular disks with openings along the axis, are spaced from the magnets so that the deflected charged particles are deposited onto the annular disks.